Device and method for comparing optical signals

Abstract

An optical comparator for comparing, bit by bit, corresponding bits of two optical digital signals which are identical in the number of constituent optical bit signals, includes an optical waveguide layer made of an acousto-optical material. A surface acoustic wave generator generates a surface acoustic wave propagating on the waveguide layer, and a photocoupler directs the constituent bit signals to be of the two optical digital signals incident on the waveguide layer so as to satisfy the Bragg diffraction condition with respect to the surface acoustic wave propagating on the waveguide layer. A photosensor detects diffracted light and/or undiffracted light which has passed through a surface acoustic wave interaction region. The position of the generator and/or the surface acoustic wave generating timing is so determined that initiation or cessation of the interaction of the surface acoustic wave with a bit of one of the optical digital signals is simultaneous with initiation or cessation of the wave interaction with a corresponding bit of the other optical bit signal.

What is claimed is: 
     
       1. An optical comparator for comparing two optical digital signals comprising: optical waveguide means, including an acousto-optical material, for propagating light therethrough;   means for coupling first and second optical digital signals to said waveguide means, said first and second signals each comprising plural optical bits, a one-to-one correspondence existing between the plural bits of said first signal and the plural bits of said second signal;   surface acoustic wave generator means for generating surface acoustic waves and for propagating said waves through the optical waveguide means to impinge upon said first and second signals propagating through said waveguide means at the Bragg angle; and   means for detecting the bits of said first signal which have been diffracted or the undiffracted bits of said first signal and for detecting the bits of said second signal which have been diffracted or the bits of said second signal which have not been diffracted,   wherein the initiation or cessation of interaction between a surface wave produced by said surface acoustic wave generator means and any given one of the plural optical bits of said first signal is simultaneous with initiation or cessation of the interaction of the surface acoustic wave with a corresponding bit of the second optical signal.   
     
     
       2. An optical comparator as defined in claim 1 wherein: surface acoustic waves produced by said generator means successively interact with the bits of said first signal and successively interact with the bits of said second signal, any bit of said first signal and the bit of said second signal corresponding thereto being diffracted at the same time; and   said detecting means includes means for detecting the diffracted light of said first optical digital signal and the undiffracted light of the second optical digital signal.   
     
     
       3. An optical comparator as defined in claim 1 wherein: surface acoustic waves produced by said generator means successively interact with the bits of said first signal and also successively interact with the bits of said second signal so that each bit of said first signal is diffracted at the same time that a corresponding bit of said second signal is undiffracted; and   said detecting means includes means for detecting one of the diffracted bits and the undiffracted bits of the first and second optical digital signals.   
     
     
       4. An optical comparator as defined in claim 1 wherein: said generator means includes means for generating first and second pulsating surface acoustic waves, said first wave successively interacting with the bits of the first signal at the same time that the second wave interacts with corresponding bits of the second signal; and   said detecting means includes means for detecting the diffracted bits of each of said first and second optical digital signals.   
     
     
       5. An optical comparator as defined in claim 1 wherein said detecting means includes: means for detecting light;   means for converting the detected light to an electric signal; and   means for differentiating the electric signal.   
     
     
       6. An optical comparator as in claim 1 wherein said detecting means includes: means for detecting light;   means for converting the detected light to an electric signal; and   means for comparing the level of the electric signal with a reference level.   
     
     
       7. An optical comparator as defined in claim 1 wherein said detecting means includes: a first detector for detecting the diffracted and/or the undiffracted bits of the first optical digital signal; and   a second detector for detecting the diffracted and/or undiffracted bits of the second optical digital signal.   
     
     
       8. An optical comparator as defined in claim 1 wherein the surface acoustic wave generator means is disposed on the optical waveguide means between the path of propagation of the first optical digital signal and the path of propagation of the second optical digital signal. 
     
     
       9. An optical comparator as defined in claim 1 wherein the surface acoustic wave generator means is disposed on said waveguide means, and said coupling means propagates said first and second optical signals through said waveguide means along first and second propagation paths respectively, said second path disposed between said first path and said generator means. 
     
     
       10. An interdigital transducer for generating a surface acoustic wave propagating through an optical waveguide to selectively diffract light propagating through said waveguide, comprising a plurality of substantially parallel electrodes and first and second common electrodes, alternate electrodes being connected to the same one of the first and second common electrodes, at least one parallel electrode positioned at at least one end of the transducer being unconnected to said common electrodes, the number of said unconnected parallel electrodes determined in accordance with the position of the transducer with respect to light propagating through said waveguide. 
     
     
       11. A method of comparing, two optical digital signals comprising: coupling first and second optical digital signals to an optical waveguide means, said waveguide means for propagating light therethrough, said first and second signals each comprising plural optical bits, the plural bits of said first signal having a one-to-one correspondence with the plural bits of said second signal;   propagating first and second surface acoustic waves through said waveguide means, said first wave successively interacting with the bits of said first signal, said second signal successively interacting with the bits of said second signal, the initiation or cessation of interaction between said first wave and any givenbit of said first optical signal being simultaneous with the initiation of cessation of interaction between said second wave and the bit of said second optical signal corresponding to the given bit of said first optical signal; and   detecting a variation produced by the interaction between said first and second signals and said first and second waves.   
     
     
       12. A method of adjusting the effective position of an interdigital transducer of the type comprising a plurality of parallel electrodes and first and second common electrodes, alternate ones of the parallel electrodes of the transducer having at least one end connected to the first common electrode, the ones of the parallel electrodes not connected to said first common electrode being connected to the second common electrode, the method comprising disconnecting a selected number of said parallel electrodes disposed at at least one end of the transducer from the first and second common electrodes originally connected thereto, and selecting the number of said parallel electrodes being disconnected by said disconnecting step in accordance with the position of the transducer with respect to a light beam to be diffracted. 
     
     
       13. An apparatus for comparing first and second optical digital values comprising: optical waveguide means for propagating light therethrough;   first transmitting means for transmitting a first optical signal comprising a first plurality of light beams over plural mutually-parallel beam paths A 1  -A n  through said waveguide means;   second transmitting means for transmitting a second optical signal comprising a second plurality of light beams over plural mutually-parallel beam paths B 1  -B n  through said waveguide means, said beams travelling over said paths B 1  -B n  having a one-to-one correspondence with said beams of said first optical signal travelling over said paths A 1  -A n  ;   means for propagating a first acoustic wavefront through said waveguide means and for causing said first wavefront to successively impinge upon said paths A 1  -A n  and for simultaneously propagating a second acoustic wavefront through said waveguide means and for causing said second wavefront to successively impinge upon said paths B 1  -B n  said second wavefront impinging or ceasing to impinge upon each path B i  of said plural paths B 1  -B n  at the same time said first wavefront impinges or ceases to impinge upon the path A i  of said paths A 1  -A n , the beam of said first signal travelling over said path A i  corresponding to the beam of said second signal travelling over said path B i  ; and   comparing means for comparing the intensity of (a) the beams of said first signal diffracted by said first wavefront or (b) the beams of said first signal undiffracted by said first wavefront with the intensity of (c) the beams of said second signal diffracted by said second wavefront or (d) the beams of said second signal undiffracted by said second wavefront.   
     
     
       14. An apparatus as in claim 13 wherein said comparing means includes means for comparing the intensity of the beams of said first signal diffracted by said first wavefront with the intensity of the beams of said second signal undiffracted by said second wavefront. 
     
     
       15. An apparatus as in claim 13 wherein said comparing means includes means for comparing the intensity of the beams of said first signal diffracted by said first wavefront with the intensity of the beams of said second signal diffracted by said second wavefront. 
     
     
       16. An apparatus as in claim 13 wherein said propagating means includes transducer means, disposed on said waveguide means between the paths A 1  -A n  and the paths B 1  -B n , for directing said first wavefront in a first direction along a line toward the propagation paths A 1  -A n  and for directing said second wavefront in a second direction opposite to said first direction along said line toward the propagation paths B 1  -B n . 
     
     
       17. An apparatus as in claim 16 wherein: said transducer means includes a first end portion for producing said first wavefront and a second end portion for producing said second wavefront; and   the distance along said line from said first end portion of said transducer means to any path A i  of said plural paths A 1  -A n  is equal to the distance along said line from said second end portion of said transducer means to the path B i  of said plural paths B 1  -B n  over which travels the beam of said second signal corresponding to the beam of said first signal travelling over said path A 1 .   
     
     
       18. An apparatus as in claim 13 wherein: said paths A 1  -A n  are adjacent and parallel to the paths B 1  -B n  ; and   said propagating means includes means for directing said first wavefront along an acoustical wavefront path toward said mutually-parallel paths A 1  -A n , B 1  -B n  and for thereafter directing said second wavefront along the same acoustic wavefront path.   
     
     
       19. An apparatus as in claim 18 wherein: said first transmitting means transmits said plural first signal beams along respective plural mutually-parallel paths A 1  -A n , each of said plural paths A 1  -A n  being spaced from the paths adjacent thereto by the same predetermined distance;   said second transmitting means transmits said plural second signal beams along respective plural mutually-parallel paths B 1  -B n  each of said plural paths B 1  -B n  being spaced the same predetermined distance from paths adjacent thereto; and   the path B 1  is parallel and adjacent to the path A n  and is spaced therefrom by said same predetermined distance.   
     
     
       20. An apparatus as in claim 19 further including timing means for controlling said transducer means to transmit said second wavefront a predetermined time after said transducer means transmits said first wavefront, said predetermined time equal to the difference between the time said first wavefront impinges said path A 1  and the time said wavefront impinges said path B 1 . 
     
     
       21. An apparatus as in claim 13 wherein said comparing means includes; detecting means for converting light to an electrical signal;   collimating means for directing one of (a) the beams of said first signal diffracted by said first wavefront and (b) the beams of said first signal undiffracted by said first wavefront and one of (c) the beams of said second signal diffracted by said second wavefront and (d) the beams of said second signal undiffracted by said second wavefront toward said detecting means;   means for differentiating said electrical signal produced by said detecting means; and   means for detecting changes in said differentiated electrical signal.   
     
     
       22. A method for comparing first and second optical digital values comprising the steps of: (1) transmitting a first optical signal comprising plural beams of light through an optical waveguide over plural mutually-parallel paths A 1  -A n  ;   (2) transmitting a second optical signal comprising plural beams of light through said waveguide over plural mutually-parallel paths B 1  -B n  said beams travelling over said paths B 1  -B n  having a one-to-one correspondence with said beams travelling over said paths A 1  -A n  ;   (3) propagating a first acoustic wavefront through said waveguide to successively impinge upon said paths A 1  -A n  ;   (4) propagating a second acoustic wavefront through said waveguide to successively impinge upon said paths B 1  -B n , said second wavefront impinging upon each path B i  of said paths B 1  -B n  at the same time said first wavefront impinges upon a path A i  of said paths A 1  -A n , the beam of said first signal travelling over said path A i  corresponding to the beam of said second signal travelling over said path B i  ;   (5) successively diffracting, with said first wavefront, beams of said first signal as said first wavefront impinges upon said paths A 1  -A n  during said propagating step (3);   (6) successively diffracting, with said second wavefront, the beams of said second signal as said second wavefront impinges upon said paths B 1  -B n  during said propagating step (4); and   (7) comparing the intensity of (a) the beams of said first signal diffracted by said diffracting step (5) or (b) the beams undiffracted by said diffracting step (5) with the intensity of (c) the beams diffracted by said diffracting step (6) or (d) the beams undiffracted by said diffracting step (6).   
     
     
       23. A method as in claim 22 wherein said comparing step (7) includes the step of comparing the intensity of the beams of said first signal diffracted by said diffracting step (5) with the intensity of the beams of said second signal not diffracted by said diffracting step (6). 
     
     
       24. A method as in claim 23 wherein said comparing step (7) includes the steps of: directing one of (a) the beams of said first signal diffracted by said diffracting step (5) and (b) the beams of said first signal not diffracted by said diffracting step (5) and one of (c) the beams of said second signal diffracted by said diffracting step (6) and (d) the beams of said second signal not diffracted by said diffracting step (6) toward a detection area;   converting the light impinging upon said detection area to an electrical signal;   differentiating said electrical signal produced by said converting step; and   detecting changes in said differentiated electrical signal.   
     
     
       25. A method as in claim 22 wherein said comparing step (7) includes the step of comparing the intensity of the beams of said first signal diffracted by said diffracting step (5) with the intensity of the beams of said second signal diffracted by said diffracting step (6). 
     
     
       26. A method as in claim 22 wherein: said propagating step (3) includes the steps of: producing said first acoustic wavefront with a transducer disposed on said waveguide between the paths A 1  -A n  and the paths B 1  -B n , and   directing said first wavefront in a first direction along a line toward the propagation paths A 1  -A n  ; and     said propagating step (4) includes the steps of: producing said second acoustic wavefront with said transducer, and   directing said second wavefront in a second direction opposite to said first direction along said line toward the propagation paths B 1  -B n .     
     
     
       27. A method as in claim 26 wherein: said first wavefront directing step includes the step of propagating said first wavefront along said line from a first end portion of said transducer to a path A i  of said plural paths A 1  -A n  over a first distance; and   said second wavefront directing step includes the step of propagating said second wavefront from a second end portion of said transducer along said line over a second distance to the path B i  of said plural paths B 1  -B n  over which travels the beam of said second signal corresponding to the beam of said first signal travelling over said path A i , said first distance equal to said second distance.   
     
     
       28. A method as in claim 22 wherein: said transmitting step (2) transmits said plural beams of said second optical signal along plural mutually-parallel paths B 1  -B n  which are also mutually-parallel to said plural mutually-parallel paths A 1  -A n , said paths B 1  -B n  adjacent to said paths A 1  -A n  ;   said propagating step (3) includes the step of directing said first wavefront along an acoustical wavefront path at a first instant in time toward said mutually-parallel paths A 1  -A n , B 1  -B n  ; and   said propagating step (4) includes the step of propagating said second wavefront along the same acoustic wavefront path at a second instant in time later than said first instant toward said mutually-parallel paths A 1  -A n , B 1  -B n .   
     
     
       29. A method as in claim 28 wherein: said transmitting step (1) transmits said plural first signal beams along respective plural mutually-parallel paths A 1  -A n , each of said plural paths A 1  -A n  being spaced from the paths adjacent thereto by the same predetermiend distance;   said transmitting step (2) includes the step of transmitting said plural second signal beams along respective plural mutually-parallel paths B 1  -B n , each of said plural paths B 1  -B n  being spaced the same predetermined distance from the paths adjacent thereto, the path B 1  parallel and adjacent to the path A n  and spaced therefrom by said same predetermined distance.   
     
     
       30. A method as in claim 29 further including the steps of: timing a time interval beginning at said first instant of time;   comparing the duration of said timed interval with a predetermined duration related to the difference between the time said first wavefront impinges said path A 1  and the time said second wavefront impinges said path B 1  ; and   said propagating step (4) includes the step of propagating said second wavefront after said comparing step determines said predetermined duration has elapsed.   
     
     
       31. A method of forming an interdigital transducer comprising the steps of: (1) photolithographically forming a conductive pattern on a substrate, said pattern comprising a plurality of parallel elongated electrodes and first and second common electrodes, alternate ones of the elongated electrodes of the transducer having at least one end connected to the first common electrode, the ones of the elongated electrodes not connected to the first common electrode being connected to the second common electrode;   (2) applying an alternating electrical signal to said pattern to thereby propagate first and second acoustic wavefronts therefrom through said substrate;   (3) measuring the difference between the time said first wavefront diffracts a first light beam propagating through said substrate and the time said second wavefront diffracts a second light beam propagating through said substrate;   (4) disconnecting an elongated electrode of said pattern from the one of the first and second common electrodes originally connected thereto; and   (5) repeating said applying step (2), said determining step (3) and said disconnecting step (4) until said determining step (3) determines said first and second beams are diffracted simultaneously.